1. Field of the Invention
The present invention relates to a center-pivoted transaxle, which is mounted onto a vehicle frame so as to be turnable around an axis oriented in a fore-and-aft direction of a vehicle, being applicable for various four-wheel-drive/two-wheel-steering (4WD/2WS) industrial vehicles and four-wheel-drive/four-wheel-steering (4WD/4WS) industrial vehicles, such as agricultural tractors, riding lawn mowers and construction vehicles. The transaxle can be provided for driving front wheels of a tractor, for example.
2. Background Art
Conventionally, there are many disclosed characteristic arts concerning various industrial vehicles equipped with a hydraulic transaxle, e.g., agricultural tractors, riding lawn mowers and construction vehicles.
Especially, Japanese Patent Application Publication No. Sho 55-132325 discloses a center-pivoted transaxle, including a pair of hydraulic motors for differentially driving respective left and right axles, typically serving as a transaxle for driving front wheels of a tractor.
In the transaxle disclosed in the document, the left and right hydraulic motors having respective vertical motor shafts are mounted upright on a top of an axle casing, in which a pair of left and right front-wheel driving shafts are supported to be drivingly connected to the respective motor shafts, thereby expanding a ground clearance of the front wheel axles.
However, the height of the left and right hydraulic motors projecting upward from the axle casing directly causes increase of the height of an entire vehicle traveling system including the transaxle. Furthermore, the hydraulic motors, disposed out of the axle casing and oriented perpendicular to the axle casing, are disadvantageous in assembly and compactness.
Also, conventionally, there are many documents disclosing arts of front-wheel acceleration for the above-mentioned working vehicles, wherein rotary speeds of front wheels are increased during turning of a vehicle so as to smoothen the turning of the vehicle.
For example, Japanese Patent Application Publication No. 2001-178208 discloses a vehicle equipped with a front-wheel transmission including a friction clutch, wherein, when front wheels are turned rightward or leftward at an angle equal to or beyond a predetermined value, the friction clutch is engaged so as to accelerate rotation of a drive shaft for front wheels faster than rotation of a drive shaft for rear wheels.
However, in this art, the front wheels are not accelerated before a manipulation degree of a steering manipulator reaches the predetermined value. Namely, the reaction of front-wheel acceleration does not cover the entire manipulation range of the steering manipulator, whereby an operator may feel discomfort. Furthermore, the front-wheel transaxle including the friction clutch complicates a vehicle configuration and increases a vehicle weight. From this viewpoint, the front-wheel transaxle has a room for improvement.
Further, the vehicle is disadvantageous to have a mid-mount mower below a middle portion thereof because the drive shaft narrows a space for attachment and maintenance of the mower. Another disadvantage of the drive shaft is to prevent lowering a footplate of the vehicle.
Further, the transmission system interposed between the drive shaft and the front wheels includes a mechanical differential unit requiring a large space in the axle casing, thereby, reducing a ground clearance.